A standard recessed valve has a housing formed with a connection face, an intake port adapted to be connected to a supply of pressurized liquid, an output port, an intake passage extending from the intake port to the connection face, and an output passage extending from the bore to the output port. A valve cartridge secured to the connection face is operable to conduct liquid between the intake passage and the bore.
When such a valve is used with, for example, a fitting like a telephone shower there is the potential of back flow from the fitting into the lines in the event of a pressure inversion. The code therefore requires that the system be fitted with a backflow preventer that vents and/or blocks flow back in case of such a pressure inversion. Such retrofitting with these extra elements is time consuming and expensive, and they take up extra space in often very cramped situations.